Bipolar field effect transistor structures and methods of forming the same

ABSTRACT

Bipolar field effect transistor (BiFET) structures and methods of forming the same are provided. In one embodiment, an apparatus includes a substrate and a plurality of epitaxial layers disposed over the substrate. The plurality of epitaxial layers includes a first epitaxial layer, a second epitaxial layer disposed over the first epitaxial layer, and a third epitaxial layer disposed over the second epitaxial layer. The first epitaxial layer includes at least a portion of a channel of a first field effect transistor (FET) and the third epitaxial layer includes at least a portion of a channel of a second FET.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/792,083, filed Jun. 2, 2010 entitled “BIPOLAR/DUAL FET STRUCTUREHAVING FETS WITH ISOLATED CHANNELS”, which is a continuation of U.S.patent application Ser. No. 12/284,804, filed Sep. 24, 2008 entitled“BIPOLAR/DUAL FET STRUCTURE INCLUDING ENHANCEMENT AND DEPLETION MODEFETS WITH ISOLATED CHANNELS”, each of which are herein incorporated byreference in their entireties.

BACKGROUND

1. Field of the Invention

The present invention generally relates to the field of semiconductorstructures. More particularly, the invention relates to transistorsemiconductor structures.

2. Description of the Related Art

By utilizing BiFET technology, bipolar transistors, such asheterojunction bipolar transistors (HBTs), and field effect transistors(FETs), such as enhancement-mode (E-mode) and depletion-mode (D-mode)FETs, can be integrated on the same semiconductor die to provideincreased circuit design flexibility. In an integrated structure, abipolar transistor, such as an HBT, an E-mode FET, and a D-mode FET caneach be advantageously tailored for specific applications. For example,an HBT, a D-mode FET, and an E-mode FET can be integrated on asubstrate, such as a semi-insulating gallium arsenide (GaAs) substrate,to form a power amplifier, a bias circuit, and a radio frequency (RF)switch, respectively, for a communications device, such as a cell phone.However, previous attempts at integrating a bipolar transistor withE-mode and D-mode FETs on a substrate have undesirably affected therespective performances of the E-mode and D-mode FETs.

In one conventional approach, for example, an HBT can be formed over asubstrate, such as a semi-insulating GaAs substrate, and E-mode andD-mode FETs can be integrated under the sub-collector of the HBT.However, in this conventional approach, the E-mode and D-mode FETstypically have shared epitaxial layers, which can undesirably affect theanalog properties of the E-mode FET. Also, as a result of the sharedepitaxial layers, coupling can occur between the E-mode and D-mode FETs,which can undesirably affect the RF switching performance of the D-modeFET. Thus, in the aforementioned conventional approach, the performanceof the E-mode FET cannot be optimized without affecting the performanceof the D-mode FET, and vice versa.

SUMMARY OF THE INVENTION

In certain embodiments, the present disclosure relates to an apparatusthat includes a substrate and a first epitaxial layer disposed over thesubstrate, the first epitaxial layer including at least a portion of achannel of a first field effect transistor (FET). The apparatus furtherincludes a second epitaxial layer disposed over the first epitaxiallayer and a third epitaxial layer disposed over the second epitaxiallayer, the third epitaxial layer including at least a portion of achannel of a second FET.

In certain embodiments, the present disclosure relates to a method ofmaking a bipolar field effect transistor structure. The method includesforming a first epitaxial layer over a substrate, the first epitaxiallayer including at least a portion of a channel of a first field effecttransistor (FET). The method further includes forming a second epitaxiallayer over the first epitaxial layer. The method further includesforming a third epitaxial layer over the second epitaxial layer, thethird epitaxial layer including at least a portion of a channel of asecond FET.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross-sectional view of an exemplary bipolar/dualFET structure in accordance with one embodiment of the presentinvention.

FIG. 2 illustrates a cross-sectional view of exemplary enhancement-modeand depletion-mode FETs in accordance with one embodiment of the presentinvention.

FIG. 3 illustrates a cross-sectional view of an exemplary bipolar/dualFET structure in accordance with one embodiment of the presentinvention.

DETAILED DESCRIPTION

The present invention is directed to bipolar/dual FET structuresincluding enhancement and depletion mode FETs with isolated channels.The following description contains specific information pertaining tothe implementation of the present invention. One skilled in the art willrecognize that the present invention may be implemented in a mannerdifferent from that specifically discussed in the present application.Moreover, some of the specific details of the invention are notdiscussed in order not to obscure the invention. The specific detailsnot described in the present application are within the knowledge of aperson of ordinary skill in the art.

The drawings in the present application and their accompanying detaileddescription are directed to merely exemplary embodiments of theinvention and are not drawn to scale. To maintain brevity, otherembodiments of the invention which use the principles of the presentinvention are not specifically described in the present application andare not specifically illustrated by the present drawings.

As will be discussed in detail below, the present invention provides aninnovative bipolar/dual FET structure including a bipolar transistor,such as a heterojunction bipolar transistor (HBT), and E-mode and D-modeFETs, wherein the E-mode FET is isolated from the D-mode FET. Althoughan exemplary bipolar/dual FET structure comprising an exemplary NPNbipolar transistor, an exemplary E-mode NFET, and an exemplary D-modeNFET are used to illustrate the present invention, the present inventionmay also apply to bipolar/dual FET structure comprising a PNP bipolartransistor, an E-mode PFET, and a D-mode PFET. Also, although GaAs(gallium arsenide) is a semiconductor material that is utilized toillustrate the present invention, the present invention may also applyto other types of semiconductor material, such as indium phosphide (InP)or gallium nitride (GaN).

FIG. 1 shows a cross-sectional view of a semiconductor die including anexemplary bipolar/dual FET structure in accordance with one embodimentof the present invention. Certain details and features have been leftout of FIG. 1, which are apparent to a person of ordinary skill in theart. In FIG. 1, structure 100 includes bipolar/dual FET structure 102 onsubstrate 104, which can be a semi-insulating GaAs substrate in oneembodiment of the present invention. In other embodiments, substrate 104can comprise indium phosphide, gallium nitride, or other type ofsemiconductor material. Bipolar/dual FET structure 102 includes bipolartransistor 106, E-mode (enhancement-mode) FET 108, and D-mode(depletion-mode) FET 110. Bipolar transistor 106 includes sub-collector112, etch stop segment 114, collector 116, base 118, emitter 120,emitter contact 122, etch stop segment 124, and emitter cap 126. Bipolartransistor 106 can comprise, for example, an NPN HBT. In one embodiment,bipolar transistor 106 may comprise a PNP HBT.

E-mode FET 108 includes back gate 128, contact regions 130 and 132, andchannel 134, which is a conductive channel and includes channel segments136, 138, and 140. E-mode FET 108 can be, for example, an NFET. In oneembodiment, E-mode FET 108 can be a PFET. E-mode FET 108 can comprise,for example, a heterostructure FET (HFET), such as a High ElectronMobility Transistor (HEMT) or a Pseudomorphic HEMT (PHEMT). In oneembodiment, E-mode FET 108 can comprise a Metal-SemiconductorSemiconductor FET (MESFET). D-mode FET 110 includes contact regions 142and 144 and channel 146, which is a conductive channel and includeschannel segment 148. In one embodiment, channel 146 of D-mode FET 110can comprise multiple channel segments. D-mode FET 110 can be, forexample, an NFET. In one embodiment, D-mode FET 110 can be a PFET.D-mode FET 110 can comprise, for example, an HFET, such as a HEMT orPHEMT. In one embodiment, D-mode FET 110 can comprise a MESFET.Bipolar/Dual FET structure 102 also includes isolation regions and base,emitter, collector, source, drain, gate, and back gate contacts, whichare not shown in FIG. 1.

Bipolar/Dual FET structure 102 can be utilized in a wirelesscommunication device, such as a cell phone, or other type of electronicdevice. Bipolar transistor 106 can be utilized, for example, as a poweramplifier in a cell phone or other electronic device. E-mode FET 108 canbe utilized, for example, in analog applications, such as bias andcontrol applications, and can also be utilized in digital logiccircuits. Although well suited for utilization in RF switchingapplications, D-mode FET 110 can also be utilized in digital logiccircuits, for example.

As shown in FIG. 1, epitaxial segment 111 and channel segment 148 aresituated over substrate 104. Epitaxial segment 111 and channel segment148 each comprise a portion of epitaxial layer 150, which can comprise,for example, GaAs in one embodiment. Channel segment 148 can be, forexample, a conductive channel segment. In one embodiment, one or morebuffer layers can be situated between channel segment 148 and substrate104. In one embodiment, channel segment 148 can comprise lightly doped Ntype GaAs. Epitaxial segment 111 and channel segment 148 can be formed,for example, by depositing epitaxial layer 150 over substrate 104 byusing a metal organic chemical vapor deposition (MOCVD) process, amolecular beam epitaxy (MBE) process or other deposition process andappropriately patterning epitaxial layer 150. Also shown in FIG. 1,sub-collector 112 is situated over a epitaxial segment 111 and contactregions 142 and 144 are situated over channel segment 148. Sub-collector112 and contact regions 142 and 144 each comprise a portion of epitaxiallayer 152, which can comprise, for example, heavily doped N type GaAs inone embodiment. Sub-collector 112 and contact regions 142 and 144 can beformed, for example, by depositing epitaxial layer 152 over epitaxiallayer 150 by using a MOCVD process, an MBE process, or other depositionprocess and appropriately patterning epitaxial layer 152.

Further shown in FIG. 1, etch stop segment 114 is situated oversub-collector 112 and epitaxial segment 115 is situated over contactregions 142 and 144. Etch stop segment 114 and epitaxial segment 115each comprise a portion of epitaxial layer 154, which can comprise, forexample, indium gallium phosphide (InGaP) in one embodiment. Etch stopsegment 114 and epitaxial segment 115 can be formed, for example, bydepositing epitaxial layer 154 over epitaxial layer 152 by using a MOCVDprocess, an MBE process, or other deposition process and appropriatelypatterning epitaxial layer 154. Also shown in FIG. 1, collector 116 issituated over etch stop segment 114 and epitaxial segment 117 issituated over epitaxial segment 115. Collector 116 and epitaxial segment117 each comprise a portion of epitaxial layer 156, which can comprise,for example, lightly doped N type GaAs in one embodiment. Collector 116and epitaxial segment 117 can be formed, for example, by depositingepitaxial layer 156 over epitaxial layer 154 by using a MOCVD process,an MBE process, or other deposition process and appropriately patterningepitaxial layer 156.

Also shown in FIG. 1, base 118 is situated over collector 116 and backgate 128 is situated over epitaxial portion 117 of epitaxial layer 156.Base 118 and back gate 128 each comprise a portion of epitaxial layer158, which can comprise, for example, heavily doped P type GaAs in oneembodiment. Base 118 and back gate 128 can be formed, for example, bydepositing epitaxial layer 158 over epitaxial layer 156 by using a MOCVDprocess, an MBE process, or other deposition process and appropriatelypatterning epitaxial layer 158. Further shown in FIG. 1, emitter 120 issituated over base 118 and channel segment 140 is situated over backgate 128. Emitter 120 and channel segment 140 each comprise a portion ofepitaxial layer 160, which can comprise lightly doped N type InGaP inone embodiment. Emitter 120 and channel segment 140 can be formed, forexample, by depositing epitaxial layer 160 over epitaxial layer 158 andappropriately patterning epitaxial layer 160. Channel segment 140 canbe, for example, a conductive channel segment.

Also shown in FIG. 1, emitter contact 122 is situated over emitter 120and channel segment 138 is situated over channel segment 140. Emittercontact 122 and channel segment 138 each comprise a portion of epitaxiallayer 162, which can comprise, for example, lightly doped N type GaAs inone embodiment. Emitter contact 122 and channel segment 138 can beformed, for example, by depositing epitaxial layer 162 over epitaxiallayer 160 by using a MOCVD process, an MBE process, or other depositionprocess and appropriately patterning epitaxial layer 162. Channelsegment 140 can be, for example, a conductive channel segment. Furthershown in FIG. 1, etch stop segment 124 is situated over emitter contact122 and channel segment 136 is situated over channel segment 138. Etchstop segment 124 and channel segment 136 each comprise a portion ofepitaxial layer 164, which can comprise, for example, lightly doped Ntype InGaP in one embodiment. Etch stop segment 124 and channel segment136 can be formed, for example, by depositing epitaxial layer 164 overepitaxial layer 162 by using a MOCVD process or other deposition processand appropriately patterning epitaxial layer 164. Channel segment 136can be, for example, a conductive channel segment.

Also shown in FIG. 1, emitter cap 126 is situated over etch stop segment124 and contact regions 130 and 132 are situated over channel segment136. Emitter cap 126 and contact regions 130 and 132 each compriseportions of epitaxial layer 166, which can comprise, for example,heavily doped N type GaAs in one embodiment. In one embodiment,epitaxial layer 166 can comprise heavily doped N type indium galliumarsenide (InGaAs). Emitter cap 126 and contact regions 130 and 132 canbe formed, for example, by depositing epitaxial layer 166 over epitaxiallayer 164 by using a MOCVD process, an MBE process, or other depositionprocess and appropriately patterning epitaxial layer 166.

In bipolar/dual FET 102, channel 134 of E-mode FET 108 is situated aboveand isolated from channel 146 of D-mode FET 110, which electrically andphysically decouples E-mode FET 108 from D-mode FET 110. By decouplingE-mode FET 108 from D-mode FET 110, E-mode FET 108 and D-mode FET 110can each be independently optimized for a particular application. Forexample, E-mode FET 108 can be optimized for analog applications, suchas bias and control applications. D-mode FET 110 can be optimized, forexample, for RF switching applications. However, E-mode FET 108 andD-mode FET 110 can also be utilized in digital logic circuits, forexample.

FIG. 2 shows a cross-sectional view of exemplary E-mode and D-mode FETssituated over a substrate in accordance with one embodiment of thepresent invention. In FIG. 2, E-mode FET 208 and D-mode FET 210correspond, respectively, to E-mode FET 108 and D-mode FET 110 inbipolar/dual FET structure 102 in FIG. 1. In particular, epitaxialsegments 215 and 217, back gate 228, contact regions 230, 232, 242, and244, channels 234 and 246, and channel segments 236, 238, 240, and 248in FIG. 2 correspond, respectively, to epitaxial segments 115 and 117,back gate 128, contact regions 130, 132, 142, and 144, channels 134 and146, and channel segments 136, 138, 140, and 148 in FIG. 1. In FIG. 2,E-mode FET 208 includes back gate 228, contact regions 230 and 232,channel 234, channel segments 236, 238, and 240, and respective backgate, source, gate, and drain contacts 272, 273, 274, and 275. D-modeFET 210 includes contact regions 242 and 244, channel 246, channelsegment 248, and respective source, gate, and drain contacts 276, 277,and 278.

As shown in FIG. 2, E-mode FET 208 is situated between isolation regions280 and 281 and D-mode FET 210 is situated between isolation regions 281and 282. Isolation regions 280, 281, and 282 are non-conductive regionsfor providing electrical isolation between adjacent transistors. In oneembodiment, isolation regions 280, 281, and 282 can each comprise aimplant-damage region. In such embodiment, isolation regions 280, 281,and 282 can each be formed by utilizing an implant process to damage theepitaxial structure of a selected portion of epitaxial layers 150 and152 in FIG. 1. In one embodiment, isolations regions 280, 281, and 282can each comprise a trench filled with, for example, nitride, polyimide,or other dielectric material. In such embodiment, isolation regions 280,281, and 282 can each be formed by etching a trench in a selectedportion of epitaxial layers 150 and 152 and filling the trench with adielectric material, such as nitride or polyimide.

Also shown in FIG. 2, channel segment 248 is situated over substrate204, gate contact 277 and contact regions 242 and 244 are situated overchannel segment 248, source contact 276 is situated over contact region242, and drain contact 278 is situated over contact region 244. Gatecontact 277 can comprise a metal or metal stack, such as a metal stackcomprising, for example, platinum-titanium-gold in one embodiment.Source contact 276 and drain contact 278 can be ohmic contacts and cancomprise, for example, gold-nickel-germanium, palladium-germanium-gold,or other metal alloy. Gate contact 277 and source and drain contacts 276and 278 can be formed, for example, by using a sputter process, anevaporation process, or other deposition process.

Further shown in FIG. 2, epitaxial segment 270 is situated oversubstrate 204 and epitaxial segment 271 is situated over epitaxialsegment 270. Epitaxial segment 270 can comprise a portion of epitaxiallayer 150 in FIG. 1 and epitaxial segment can comprise a portion ofepitaxial layer 152 in FIG. 1. Also shown in FIG. 2, epitaxial segment215 is situated over epitaxial segment 271, epitaxial segment 217 issituated over epitaxial segment 215, back gate 228 is situated overepitaxial segment 217, and back gate contact 272 and channel segment 240are situated on back gate 228. Back gate contact 272 can comprise forexample, titanium-platinum-gold, platinum-titanium-platinum-gold, orother metal alloy. Back gate contact 272 can be formed, for example, byusing a sputter process, an evaporation process, or other depositionprocess.

Further shown in FIG. 2, channel segment 238 is situated over channelsegment 240, channel segment 236 is situated over channel segment 238,contact regions 230 and 232 and gate contact 274 are situated overchannel segment 236, source contact 273 is situated over contact region230, and drain contact 275 is situated over contact region 232. Gatecontact 274 is substantially similar to gate contact 277 in compositionand formation and source and drain contacts 273 and 275 aresubstantially similar to source and drain contacts 276 and 278 incomposition and formation.

As shown in FIG. 2, channel 246 of D-mode FET 210 is situated belowchannel 234 of E-mode FET 208 and isolation region 281 is situatedbetween D-mode FET 210 and E-mode FET 208. As a result, channel 246 ofD-mode FET 210 is isolated from channel 234 of E-mode FET 208, whichadvantageously decouples D-mode FET 210 from E-mode FET 208.

FIG. 3 shows a cross-sectional view of a semiconductor die including anexemplary bipolar/dual FET structure in accordance with one embodimentof the present invention. Certain details and features have been leftout of FIG. 3, which are apparent to a person of ordinary skill in theart. In FIG. 3, structure 300 includes bipolar/dual FET structure 302 onsubstrate 304. Structure 300 in FIG. 3 corresponds to structure 100 inFIG. 1. In particular, bipolar transistor 306, E-mode FET 308, D-modeFET 310, epitaxial segments 311, 315 and 317, sub-collector 312, etchstop segments 314 and 324, collector 316, base 318, emitter 320, emittercontact 322, emitter cap 326, back gate 328, contact regions 330, 332,342, and 344, channels 334 and 346, channel segments 336, 338, 340, and348, and epitaxial layers 350, 352, 354, 356, 358, 360, 362, 364, and366 in FIG. 3 correspond, respectively, to bipolar transistor 106,E-mode FET 108, D-mode FET 110, epitaxial segments 111, 115 and 117,sub-collector 112, etch stop segments 114 and 124, collector 116, base118, emitter 120, emitter contact 122, emitter cap 126, back gate 128,contact regions 130, 132, 142, and 144, channels 134 and 146, channelsegments 136, 138, 140, and 148, and epitaxial layers 150, 152, 154,156, 158, 160, 162, 164, and 166 in FIG. 1.

Also, E-mode FET 308 and D-mode FET 310 in FIG. 3 correspond,respectively, to E-mode FET 208 and D-mode FET 210 in FIG. 2. Inparticular, epitaxial segments 315, 317, 370, and 371, back gate 328,contact regions 330, 332, 342, and 344, channels 334 and 346, channelsegments 336, 338, 340, and 348, source contacts 373 and 376, gatecontacts 374 and 377, drain contacts 375 and 378, and isolation regions380, 381, and 382 in FIG. 3 correspond, respectively, to epitaxialsegments 215, 217, 270, and 271, back gate 228, contact regions 230,232, 242, and 244, channels 234 and 246, channel segments 236, 238, 240,and 248, source contacts 273 and 276, gate contacts 274 and 277, draincontacts 275 and 278, and isolation regions 280, 281, and 282 in FIG. 2.

As shown in FIG. 3, bipolar transistor 306, which can comprise, forexample, an HBT, is situated between isolation regions 379 and 380,E-mode FET 308 is situated between isolation regions 380 and 381, andD-mode FET 310 is situated between isolation regions 381 and 382.Isolation regions 380, 381, and 382 correspond, respectfully toisolation regions 280, 281, and 282 in FIG. 2, and isolation region 279is substantially similar in composition and formation to isolationregions 280, 281, and 282. Also shown in FIG. 3, epitaxial segment 311is situated over substrate 304, sub-collector 312 is situated overepitaxial layer 311 and etch stop segment 314 and collector contact 386are situated over sub-collector 312. Collector contact 386 issubstantially similar in composition and formation to source contact 376and drain contact 378, which correspond, respectively, to source contact276 and drain contact 278 in FIG. 2.

Further shown in FIG. 3, collector 316 is situated over etch stopsegment 314, base 318 is situated over collector 316 and emitter 320 andbase contacts 384 and 385 are situated over base 318. Base contacts 384and 385 are substantially similar in composition and formation to backgate contact 372, which corresponds to back gate contact 272 in FIG. 2.Also shown in FIG. 3, emitter contact 322 is situated over emitter 320,etch stop segment 324 is situated over emitter contact 322, emitter cap326 is situated over etch stop segment 324 and emitter contact 383 issituated over emitter cap 326. Emitter contact 383 is substantiallysimilar in composition and formation to source contact 373 and draincontact 375, which correspond, respectively, to source contact 273 anddrain contact 275 in FIG. 2.

Further shown in FIG. 3, epitaxial segment 370 is situated oversubstrate 304, epitaxial segment 371 is situated over epitaxial segment370, epitaxial segment 315 is situated over epitaxial segment 371,epitaxial segment 317 is situated over epitaxial segment 315, back gate328 is situated over epitaxial segment 317, and back gate contact 372and channel segment 340 are situated over back gate 328. Also shown inFIG. 3, channel segment 338 is situated over channel segment 340,channel segment 336 is situated over channel segment 338, contactregions 330 and 332 and gate contact 374 are situated over channelsegment 336, source contact 373 is situated over contact region 330, anddrain contact 375 is situated over contact region 332. Further shown inFIG. 3, channel segment 348 is situated over substrate 304, contactregions 342 and 344 and gate contact 377 are situated over channelsegment 348, source contact 376 is situated over contact region 342, anddrain contact 378 is situated over contact region 344.

In bipolar/dual FET structure 302, E-mode FET 308 can be controlled bygate contact 374 and/or back gate contact 372. In one embodiment, E-modeFET 308 can be only controlled by gate contact 374. In anotherembodiment, E-mode FET 308 can be only controlled by back gate contact372. In bipolar/dual FET structure 302, channel 334 of E-mode FET issituated above base 318 of bipolar transistor 306 and channel 346 ofD-mode FET 310 is situated below sub-collector 312 of bipolar transistor306. Thus, channel 346 of D-mode FET 310 is situated below channel 334of E-mode FET 308, which isolates channel 346 of D-mode FET 310 fromchannel 334 of the E-mode FET. Also, E-mode FET 308 is isolated fromD-mode FET 310 by isolation region 381.

By isolating channel 346 of D-mode FET 310 from channel 334 of E-modeFET 308 in bipolar/dual FET structure 302, channel 346 of D-mode FET 310is decoupled, both electrically and physically, from channel 334 ofE-mode FET 308. By decoupling channel 346 of D-mode FET 310 from channel334 of E-mode FET 308, D-mode FET 310 is decoupled from E-mode FET 308.As a result, E-mode FET 308 and D-mode FET 310 can each beadvantageously optimized for particular applications independently ofeach other. For example, E-mode FET 308 can be optimized for logic andanalog control applications, while D-mode FET 310 can be optimized forRF switching applications.

In contrast, in a conventional structure having E-mode and D-mode FETssituated below a bipolar transistor sub-collector, the performance ofthe E-mode FET cannot be optimized without undesirably affecting theperformance of the D-mode FET, and vice versa. Thus, by forming anE-mode FET over a D-mode FET, where the E-mode FET is decoupled from theD-mode FET, an embodiment of the invention provides a bipolar/dual FETstructure having increased design flexibility compared to a conventionalstructure having E-mode and D-mode FETs situated under a bipolartransistor sub-collector.

Thus, as discussed above, an embodiment of the invention provides abipolar/dual FET structure including an E-mode FET having a channelsituated above a base of a bipolar transistor, such as an HBT, and aD-mode FET having a channel situated below a sub-collector of thebipolar transistor. As a result, an embodiment of the invention providesa bipolar/dual FET structure having an E-mode and D-mode FETs that areelectrically and physically decoupled from each other, whichadvantageously enables the E-mode FET and the D-mode FET to beindependently optimized for specific applications. As a result, theinvention provides a bipolar/dual FET structure having increased designflexibility.

From the above description of embodiments of the present invention it ismanifest that various techniques can be used for implementing theconcepts of the present invention without departing from its scope.Moreover, while the present embodiments of the invention have beendescribed with specific reference to certain embodiments, a person ofordinary skill in the art would appreciate that changes can be made inform and detail without departing from the spirit and the scope of theinvention. Thus, the described embodiments are to be considered in allrespects as illustrative and not restrictive. It should also beunderstood that the invention is not limited to the particularembodiments described herein but is capable of many rearrangements,modifications, and substitutions without departing from the scope of theinvention.

What is claimed is:
 1. An apparatus comprising: a substrate; a firstepitaxial layer disposed over the substrate, the first epitaxial layerincluding at least a portion of a channel of a first field effecttransistor (FET); a second epitaxial layer disposed over the firstepitaxial layer; and a third epitaxial layer disposed over the secondepitaxial layer, the third epitaxial layer including at least a portionof a channel of a second FET, the second epitaxial layer including abase of a bipolar transistor.
 2. The apparatus of claim 1 wherein thesecond epitaxial layer further includes a back gate of the second FET.3. The apparatus of claim 1 wherein the third epitaxial layer furtherincludes an emitter of the bipolar transistor.
 4. The apparatus of claim3 further comprising a fourth epitaxial layer disposed between the firstepitaxial layer and the second epitaxial layer, the fourth epitaxiallayer including a collector of the bipolar transistor.
 5. The apparatusof claim 4 further comprising a fifth epitaxial layer disposed betweenthe first epitaxial layer and the fourth epitaxial layer, the fifthepitaxial layer including a sub-collector of the bipolar transistor. 6.The apparatus of claim 5 wherein the fifth epitaxial layer furtherincludes a contact region electrically connected to the channel of thefirst FET.
 7. The apparatus of claim 6 further comprising an isolationregion disposed between the contact region and sub-collector of thebipolar transistor.
 8. The apparatus of claim 5 further comprising asixth epitaxial layer disposed over the third epitaxial layer, the sixthepitaxial layer including an emitter contact region of the bipolartransistor.
 9. The apparatus of claim 8 wherein the sixth epitaxiallayer further includes a portion of the channel of the second FET. 10.An apparatus comprising: a substrate; a first epitaxial layer disposedover the substrate, the first epitaxial layer including at least aportion of a channel of a first field effect transistor (FET); a secondepitaxial layer disposed over the first epitaxial layer; and a thirdepitaxial layer disposed over the second epitaxial layer, the thirdepitaxial layer including at least a portion of a channel of a secondFET, the first FET being a depletion mode FET and the second FET beingan enhancement mode FET.
 11. An apparatus comprising: a substrate; afirst epitaxial layer disposed over the substrate, the first epitaxiallayer including at least a portion of a channel of a first field effecttransistor (FET); a second epitaxial layer disposed over the firstepitaxial layer; and a third epitaxial layer disposed over the secondepitaxial layer, the third epitaxial layer including at least a portionof a channel of a second FET, the substrate being a gallium arsenide(GaAs) substrate.
 12. A method of making a bipolar field effecttransistor structure, the method comprising: forming a first epitaxiallayer over a substrate, the first epitaxial layer including at least aportion of a channel of a first field effect transistor (FET); forming asecond epitaxial layer over the first epitaxial layer; and forming athird epitaxial layer over the second epitaxial layer, the thirdepitaxial layer including at least a portion of a channel of a secondFET, forming the second epitaxial layer including forming a base of abipolar transistor.
 13. The method of claim 12 wherein forming thesecond epitaxial layer further includes forming a back gate of thesecond FET.
 14. The method of claim 12 wherein forming the thirdepitaxial layer includes forming an emitter of the bipolar transistor.15. The method of claim 14 further comprising forming a fourth epitaxiallayer between the first epitaxial layer and the second epitaxial layer,the fourth epitaxial layer including a collector of the bipolartransistor.
 16. The method of claim 15 further comprising forming afifth epitaxial layer between the first epitaxial layer and the fourthepitaxial layer, the fifth epitaxial layer including a sub-collector ofthe bipolar transistor.
 17. The method of claim 16 further comprisingforming a sixth epitaxial layer over the third epitaxial layer, thesixth epitaxial layer including an emitter contact region of the bipolartransistor.
 18. The method of claim 17 wherein the sixth epitaxial layerfurther includes a portion of the channel of the second FET.